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in the preparations, the nitrobenzene is converted by suc-
cessive steps^ into azoxy-, azo- and hydrazo-benzene ; but, by
suitably modifying the conditions, the intermediate steps may
be omitted. Thus, nitrobenzene may be converted with alcoholic
caustic soda and zinc dust directly into hydrazobenzene.
If the  reduction  of   nitrobenzene  takes place   in   neutral
solution with zinc dust and water in presence of a little calcium
or ammonium chloride, or with aluminium-mercury couple and
water, /^-phenylhydroxylamine is formed (see Prep. 52, p. 148).
C0II5NO2 + 2H3 = C6H5NIIOH + HoO.
Reduction in acid solution produces an amine (see Prep. 53,
p. 149). The mechanism of the change, although giving rise to
such different products when carried out in alkaline, neutral, or
acid solution, is not essentially different in the three cases. The
first reduction product is nitrosobenzene, CGH5NO, followed by
that of p-pheriylhydroxylamine. In alkaline, solution the two
compounds unite with elimination of water to form azoxybenzene,
which may undergo further reduction in a normal fashion giving
rise to azo- and hydrazo-benzene. In acid solution, on the
other hand, p.henylhydroxylamine does not combine with nitroso-
benzene and can then undergo further reduction. The reduction
of nitrobenzene in alkaline and neutral solution is also effected,
as already described, by electrolysing the liquid in contact with
the negative electrode. If the process is conducted in presence
of concentrated sulphuric acid /-aminophenol is obtained
(Gattermann). The latter is produced by intramolecular change
from phcnylhydroxylamine, which is first formed,
C6H0NHOH = OIICGH4NH.2.    '
Azobenzene, though not a colouring matter, may be regarded
as the mother substance of the large family of azo-colours,
which are, however, prepared by a totally distinct method, viz.,
by the action of a diazo-salt on a phenol or base (see Prep. 62,
p. 163). The intramolecular change from hydrazobenzene to
benzidine is one of great technical importance. The change
occurs by the transfer of the link between the two nitrogen
atoms to the two carbon atoms in the/^nz-position,